<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>

    /*px % em rem vw vh*/
    html{
      font-size: 10px;
    }

  </style>
</head>
<body>
  <script>
    function setRemUnit(){
      var doc=document.documentElement;
      var viewWith=doc.clientWidth
      doc.style.fontSize=viewWith/375*10+'px';
    }
  </script>
</body>
</html>